Pellicle and pellicle assembly

ABSTRACT

The invention relates to a pellicle assembly comprising a pellicle frame defining a surface onto which a pellicle is attached. The pellicle assembly comprises one or more three-dimensional expansion structures that allow the pellicle to expand under stress. The invention also relates to a pellicle assembly for a patterning device comprising one or more actuators for moving the pellicle assembly towards and way from the patterning device.

This application is a continuation of U.S. patent application Ser. No.16/614,815 which was filed on Nov. 19, 2019, now allowed, which is aU.S. national phase entry of PCT/EP2018/065127 which was filed Jun. 8,2018, which claims the benefit of priority of European PatentApplication No. 17176205.7, which was filed on Jun. 15, 2017 and ofEuropean Patent Application No. 17190503.7, filed on Sep. 12, 2017, eachof the foregoing applications is incorporated herein in its entirety byreference.

FIELD

The present disclosure relates to a pellicle and a pellicle assembly. Apellicle assembly may comprise a pellicle and a frame for supporting thepellicle. A pellicle may be suitable for use with a patterning devicefor a lithographic apparatus. The present disclosure has particular, butnot exclusive, use in connection with EUV lithographic apparatus and EUVlithographic tools.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may for example project a pattern from a patterning device(e.g. a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features whichcan be formed on that substrate. A lithographic apparatus which uses EUVradiation, being electromagnetic radiation having a wavelength withinthe range 4-20 nm, may be used to form smaller features on a substratethan a conventional lithographic apparatus (which may for example useelectromagnetic radiation with a wavelength of 193 nm).

A pattern may be imparted to a radiation beam in a lithographicapparatus using a patterning device. A patterning device may beprotected from particle contamination by a pellicle. The pellicle may besupported by a pellicle frame.

The use of pellicles in lithography is well-known and well-established.A typical pellicle in a DUV lithographic apparatus is a membrane whichis located away from the patterning device and is out of the focal planeof a lithographic apparatus in use. Because the pellicle is out of thefocal plane of the lithographic apparatus, contamination particles whichland on the pellicle are out of focus in the lithographic apparatus.Consequently, images of the contamination particles are not projectedonto the substrate. If the pellicle were not present, then acontamination particle which landed on the patterning device would beprojected onto the substrate and would introduce a defect into theprojected pattern.

It may be desirable to use a pellicle in an EUV lithographic apparatus.EUV lithography differs from DUV lithography in that it is typicallyperformed in a vacuum and the patterning device is typically reflectiverather than being transmissive.

It is desirable to provide a pellicle and/or a pellicle assembly whichovercomes or mitigates one or more problems associated with the priorart. Embodiments of the invention which are described herein may haveuse in an EUV lithographic apparatus. Embodiments of the invention mayalso have use in a DUV lithographic apparatus or another form oflithographic apparatus.

SUMMARY

According to an aspect described herein, there is provided a pellicleassembly. The pellicle assembly comprises a pellicle frame defining asurface onto which a pellicle is attached. The pellicle assembly furthercomprises one or more three-dimensional expansion structures that allowthe pellicle to expand under stress.

In this way, the lifetime and the reliability of the pellicle assemblymay be increased.

At least one of the three-dimensional expansion structures may be formedwithin the pellicle frame and thereby imparted to the pellicle. Forexample, by forming the three-dimensional expansion structures withinthe pellicle frame and depositing the pellicle atop the pellicle frame,the three-dimensional expansion structures may also be imparted to thepellicle.

At least one of the three-dimensional expansion structures may form aspring. For example, a spring may comprise at least one V-shapeformation formed within the pellicle frame. The at least one structuremay comprise a plurality of springs formed within the pellicle frame andpositioned around a central portion of the pellicle. The central portionof the pellicle is a portion of the pellicle that is not in directcontact with the frame. In this way, control of the stress within thepellicle may be more accurately controlled. The at least one spring maybe a leaf-spring.

At least one of the three-dimensional expansion structures may bepresent on the central portion of the pellicle.

The frame may comprise a substrate and the pellicle assembly maycomprise at least one spring layer between the substrate and thepellicle. At least one spring may be formed within the spring layer. Theadditional of a spring layer further increases the ability to select adesired stress within the pellicle.

At least one three-dimensional expansion structure may comprise aherringbone pattern. The herringbone pattern may extend across an entirepellicle or on an outer (e.g., non-image field) portion of a pellicle.

The at least one three-dimensional expansion structure may result inroughness across a surface of the pellicle.

According to an aspect described herein, there is provided a pellicleassembly comprising a pellicle frame defining a surface onto which apellicle is attached. The surface comprises at least one adhesiveboundary for reducing adhesive spread. The provision of an adhesiveboundary reduces the tendency for adhesive to spread and thereforereduces contamination of the space between a patterning device and thepellicle.

The at least one adhesive boundary may comprise a circular boundary.Additionally or alternatively, the at least one adhesive boundary maycomprise a line boundary. By providing a circular boundary, the adhesivemay be applied within the circular boundary, causing the adhesive tospread and center within the circular boundary but reducing an amount ofadhesive that extends beyond the circular boundary. In this way, theaccuracy with which adhesive may be applied is beneficially increased.

The line boundary may be positioned adjacent an edge of the pellicleframe, the edge of the frame adjacent a central portion of the pellicle,where the central portion of the pellicle is a portion of the pelliclethat is not in direct contact with the pellicle frame.

The line boundary may be positioned between the circular boundary and acentral portion of the pellicle.

The at least one boundary may comprise a groove within the frame.

The pellicle assembly may comprise adhesive that is substantiallyconcentric with the circular boundary.

According to an aspect described herein, there is provided a pellicleassembly comprising a pellicle frame, a pellicle, and one or moreactuators for moving the pellicle assembly towards and way from apatterning device. In this way, the space between the pellicle and apatterning device may be opened and closed to allow processing of thatspace. For example, when open, the space may be flushed, while whenclosed, the space may be sealed and pressure controlled.

The actuators may be configured to transition the pellicle assemblybetween a closed configuration in which a substantially sealed volume isformed between the pellicle and a patterning device and an openconfiguration in which a volume between the pellicle and the patterningdevice is in fluid communication with an ambient environment.

According to an aspect described herein, there is provided a pellicleassembly comprising a pellicle frame defining a surface onto which thepellicle is attached. The pellicle frame comprises a first materialhaving a first coefficient of thermal expansion (CTE) and a secondmaterial having a second coefficient of thermal expansion. In this way,the overall CTE of the pellicle frame may be adjusted and selected so asto reduce (or increase) a difference over the CTE of the pellicle. Thedifference in CTE between the pellicle and the pellicle frame results instress within the pellicle after processing (such as annealing) duringmanufacture. By controlling the overall CTE of the pellicle frame, theamount of stress within the pellicle may be controlled.

The first material may comprise silicon. The first material may comprisea plurality of perforations and the second material is located withinthe plurality of perforations. In other embodiments, the first materialmay comprise one or more channels, and the second material may beprovided within the one or more channels.

The second material may at least partially surround the first material.

The second material may comprise a metal. For example, the secondmaterial may comprise aluminum and/or molybdenum.

According to an aspect described herein, there is provide a pellicleassembly comprising a pellicle frame defining a surface onto which thepellicle is attached, wherein the pellicle frame is bonded to thepellicle. The pellicle may be an annealed pellicle and the frame may bebonded to the pellicle after such annealing.

The pellicle frame may comprise a material having a coefficient ofthermal expansion (CTE) which is lower than the CTE of silicon. Forexample, the pellicle frame may comprise a glass-ceramic material suchas ZERODUR®.

The pellicle frame may have been bonded to the pellicle using a bondingprocedure operating at a temperature of less than approximately 160degrees Celsius. In this way, a pellicle frame comprising silicon may beused, while achieving a non-operational stress (i.e. pre-stress) withinthe pellicle of approximately 200 MPa. The pellicle frame may have beenbonded to the pellicle using at least one of optical contact bonding,hydrogen bonding, gold diffusion bonding or anodic bonding. It will beappreciated that any other bonding method may be used, where aparticular bonding method operates at a temperature which gives rise toa desired pre-stress within the pellicle after manufacture. By way ofexample only, other examples of bonding include, mechanical (e.g. bolts,fasteners, etc.), ceramic green body bonding (in which two pieces of aceramic material which are in the green state are bonded together andchange into a monolithic part during sintering and for which anothercommon term is co-fire bonding), direct bonding, glass bonding, atomicdiffusion bonding, and laser ablation assisted bonding.

The pellicle frame may be formed in a single piece. That is, thepellicle frame may be such that it does not comprise a plurality ofpieces which have been connected (e.g. via an adhesive or mechanicalretaining means) so as to form a whole. In this way, a number ofinterfaces between components of the pellicle assembly is reduced,thereby reducing particulate contamination.

The pellicle frame may comprise an inert coating to reduce outgassing.

The pellicle of any of the above aspects may comprise molybdenumdisilicide (MoSi₂). A stress within the pellicle may be in a range offrom 100 MPa to 250 MPa, for example approximately 200 MPa at roomtemperature.

The pellicle may comprise a graphite-based material. A stress within thepellicle may be in a range of from 300 MPa to 450 MPa, for exampleapproximately 400 MPa at room temperature.

According to an aspect described herein, there is provided alithographic apparatus arranged to project a pattern from a patterningdevice onto a substrate. A pellicle according to any preceding aspectsis positioned in the vicinity of the patterning device to preventparticles from contacting the patterning device.

According to an aspect described herein, there is provided a method ofmanufacturing a pellicle assembly. The method comprises depositing apellicle onto a substrate, bonding a pellicle frame to the pellicle suchthat the pellicle is between the pellicle frame and the substrate,etching the substrate to leave the pellicle and the pellicle frame.

The pellicle frame may be bonded to the pellicle at a temperature below160 degrees Celsius.

The method may further comprise annealing the pellicle after depositingthe pellicle onto a substrate and before bonding a pellicle frame to thepellicle.

According to an aspect described herein, there is provided a pellicleassembly, comprising a pellicle frame defining a surface onto which apellicle is attached, wherein a computer readable and writeable trackingdevice is provided on or in the pellicle frame.

The tracking device may be configured to store a unique identifier ofthe pellicle and/or the pellicle assembly.

The tracking device may be configured to store operational dataindicating usage history of the pellicle and/or information specific forthat pellicle such as parameters measured offline (e.g. transmission,reflectivity, etc.).

According to an aspect described herein, there is provided alithographic apparatus arranged to project a pattern from a patterningdevice onto a substrate, comprising a controller comprising a processorconfigured to execute computer readable instructions to cause atransceiver arrangement to read from and write to a tracking device of apellicle assembly according to the eighth aspect.

The computer readable instructions may be configured to cause theprocessor to obtain one or more operational data items from the trackingdevice and to determine whether the one or more operational data itemsexceed a threshold.

The computer readable instructions may be configured to cause theprocessor, in response to determining that the one or more operationaldata items exceed a threshold, to cause the lithographic apparatus tounload the pellicle assembly.

The computer readable instructions may be configured to cause theprocessor to cause the transceiver arrangement to write to the trackingdevice data indicating that the pellicle assembly has been unloaded.

The computer readable instructions may be configured to cause theprocessor, in response to determining that the one or more operationaldata items do not exceed the threshold, to cause the transceiver torecord operational data on the tracking device during use of thelithographic apparatus.

According to an aspect described herein, there is provided a pellicleassembly, the pellicle assembly comprising a pellicle frame defining asurface onto which a pellicle is attached, wherein the pellicle isconfigured to, in use, exhibit one or more wrinkles, wherein the one ormore wrinkles are configured to reflect a portion of an incidentradiation beam away from a substrate.

The pellicle may be configured such that the wrinkles create a maximumangle of greater than 35 mrad to a plane defined by the surface of thepatterning device.

The pellicle may be configured such that the wrinkles create a maximumangle of less than 300 mrad to a plane defined by the surface of thepatterning device.

The pellicle may be configured to reflect approximately 0.4% of anincident radiation beam during use.

According to an aspect described herein, there is provided a pellicleassembly, comprising a pellicle frame defining a surface onto which apellicle is attached and one or more tensile layers provided on asurface of the pellicle and extending inwardly beyond an inner edge ofthe pellicle frame. The one or more tensile layers act to bothstrengthen the pellicle assembly and to maintain tension in thepellicle.

At least one of the one or more tensile layers may be provided on a topside of the pellicle such that the pellicle is positioned between thetensile layer and the frame. At least one of the one or more tensilelayers may be provided on an underside of the pellicle. The one or moretensile layers may comprises a first tensile layer provided on a topside of the pellicle and a second tensile layer provided on an undersideof the pellicle.

At least one of the one or more tensile layers comprises a first portionextending inwardly from the inner edge of the pellicle frame and asecond portion extending outwardly from the inner edge of the pellicleframe, wherein the second portion is longer than the first portion. Inthis way, a majority of the tensile layer may be in communication with(either directly or through the pellicle) the pellicle frame, therebyfurther strengthening the pellicle assembly.

At least one of the one or more tensile layers may comprise amulti-layer structure.

According to an aspect described herein, there is provided alithographic apparatus arranged to project a pattern from a patterningdevice onto a substrate, comprising a pellicle assembly according to theeleventh aspect.

According to an aspect described herein, there is provided a dynamic gaslock for a lithographic apparatus, comprising a pellicle assemblyaccording to any one of the pellicle assemblies set out above.

It will be appreciated that features described in connection with one ormore of the aspects described above may be utilized in combination withother aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 schematically depicts a lithographic system comprising alithographic apparatus including a pellicle assembly;

FIG. 2 schematically depicts a pellicle assembly;

FIGS. 3a-3c schematically depict stages of construction of a pellicleassembly having expansion structures to allow expansion of a pellicleunder stress;

FIG. 4 schematically depicts the pellicle of FIG. 3 in top-down view;

FIG. 5 is a graph showing how stress in a pellicle varies with stiffnessof the expansion structures shown in FIG. 3;

FIGS. 6a-6d schematically depict stages of construction of a pellicleassembly having different expansion structures;

FIG. 7 schematically depicts a pellicle assembly having a plurality ofexpansion structures;

FIG. 8 depicts a substrate etched to provide expansion structures whichact in two dimensions;

FIGS. 9a and 9b depict views of a substrate provided with herringboneexpansion structures;

FIG. 10 schematically depicts a pellicle assembly having herringboneexpansion structures along an edge portion of the pellicle assembly;

FIG. 11 schematically depicts a pellicle assembly having herringboneexpansion structures across a surface of a pellicle;

FIG. 12 schematically depicts a pellicle having herringbone expansionstructures;

FIGS. 13a-13c schematically depict mechanical bending of a pellicleassembly to reduce tension of a pellicle;

FIGS. 14a-14c schematically depict mechanical bending of a pellicleassembly to increase tension of a pellicle;

FIG. 15 schematically depicts construction of a pellicle havingexpansion structures provided by surface roughness;

FIG. 16 schematically depicts an alternative construction of a pelliclehaving expansion structures provided by surface roughness;

FIG. 17 schematically depicts a pellicle frame comprising an adhesiveboundary;

FIGS. 18a and 18b depict views of a pellicle frame comprising anadhesive boundary;

FIG. 19a depicts a pellicle assembly in an open configuration;

FIG. 19b depicts a pellicle assembly in a closed configuration;

FIG. 19c depicts the pellicle assembly of FIG. 19b after flushing asurrounding volume;

FIGS. 20a-20c schematically depict stages of manufacturing a pellicleframe according to an embodiment;

FIGS. 21a-21b schematically depict alternative examples of a pellicleframe;

FIG. 22 schematically depicts a pellicle assembly having a pellicleframe according to an embodiment;

FIGS. 23a-23b schematically depict stages of manufacturing a pellicleassembly according to another embodiment;

FIG. 24 is a graph indicating how a pre-stress within a pellicle changeswith respect to a bonding temperature in the method depicted in FIG. 23;

FIG. 25 schematically depicts reflection of radiation from a pellicletowards a substrate;

FIG. 26 schematically depicts reflection of radiation from a wrinkledpellicle away from a substrate; and

FIGS. 27a-27c schematically depict example arrangements of a pellicleassembly strengthened by application of at least one tensile layer.

DETAILED DESCRIPTION

FIG. 1 shows a lithographic system including a pellicle assembly 15according to one embodiment described herein. The lithographic systemcomprises a radiation source SO and a lithographic apparatus LA. Theradiation source SO is configured to generate an extreme ultraviolet(EUV) radiation beam B. The lithographic apparatus LA comprises anillumination system IL, a support structure MT configured to support apatterning device MA (e.g. a mask), a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the radiation beam B before it isincident upon the patterning device MA. The projection system isconfigured to project the radiation beam B (now patterned by the maskMA) onto the substrate W. The substrate W may include previously formedpatterns. Where this is the case, the lithographic apparatus aligns thepatterned radiation beam B with a pattern previously formed on thesubstrate W.

The radiation source SO, illumination system IL, and projection systemPS may all be constructed and arranged such that they can be isolatedfrom the external environment. A gas at a pressure below atmosphericpressure (e.g. hydrogen) may be provided in the radiation source SO. Avacuum may be provided in illumination system IL and/or the projectionsystem PS. A small amount of gas (e.g. hydrogen) at a pressure wellbelow atmospheric pressure may be provided in the illumination system ILand/or the projection system PS.

The radiation source SO may take any form, and may be for example a typewhich may be referred to as a laser produced plasma (LPP) source. In analternative example, the radiation source SO may comprise one or morefree electron lasers. The one or more free electron lasers may beconfigured to emit EUV radiation that may be provided to one or morelithographic apparatus.

The radiation beam B passes from the radiation source SO into theillumination system IL, which is configured to condition the radiationbeam. The illumination system IL may include a facetted field mirrordevice 10 and a facetted pupil mirror device 11. The faceted fieldmirror device 10 and faceted pupil mirror device 11 together provide theradiation beam B with a desired cross-sectional shape and a desiredangular distribution. The radiation beam B passes from the illuminationsystem IL and is incident upon the patterning device MA held by thesupport structure MT. The patterning device MA is protected by apellicle 19, which is held in place by a pellicle frame 17. The pellicle19 and the pellicle frame 17 together form the pellicle assembly 15. Thepatterning device MA (which may for example be a mask) reflects andpatterns the radiation beam B. The illumination system IL may includeother mirrors or devices in addition to or instead of the faceted fieldmirror device 10 and faceted pupil mirror device 11.

Following reflection from the patterning device MA the patternedradiation beam B enters the projection system PS. The projection systemcomprises a plurality of mirrors which are configured to project theradiation beam B onto a substrate W held by the substrate table WT. Theprojection system PS may apply a reduction factor to the radiation beam,forming an image with features that are smaller than correspondingfeatures on the patterning device MA. A reduction factor of 4 may forexample be applied. The patterned radiation beam that is incident uponthe substrate W may comprise a band of radiation. The band of radiationmay be referred to as an exposure slit. During a scanning exposure, themovement of the substrate table WT and the support structure MT may besuch that the exposure slit travels over an exposure field of thesubstrate W. Although the projection system PS has two mirrors in FIG.1, the projection system may include any number of mirrors (e.g. sixmirrors).

The radiation sources SO shown in FIG. 1 may include components whichare not illustrated. For example, a spectral filter may be provided inthe radiation source. The spectral filter may be substantiallytransmissive for EUV radiation but substantially blocking for otherwavelengths of radiation such as infrared radiation.

As was described briefly above, the pellicle assembly 15 includes apellicle 19 that is provided adjacent to the patterning device MA. Thepellicle 19 is provided in the path of the radiation beam B such thatradiation beam B passes through the pellicle 19 both as it approachesthe patterning device MA from the illumination system IL and as it isreflected by the patterning device MA towards the projection system PS.The pellicle 19 comprises a thin film that is substantially transparentto EUV radiation (although it will absorb a small amount of EUVradiation). The pellicle 19 acts to protect the patterning device MAfrom particle contamination. The pellicle 19 may be herein referred toas an EUV transparent pellicle.

Whilst efforts may be made to maintain a clean environment inside thelithographic apparatus LA, particles may still be present inside thelithographic apparatus LA. In the absence of a pellicle 19, particlesmay be deposited onto the patterning device MA. Particles on thepatterning device MA may disadvantageously affect the pattern that isimparted to the radiation beam B and therefore the pattern that istransferred to the substrate W. The pellicle 19 provides a barrierbetween the patterning device MA and the environment in the lithographicapparatus LA in order to prevent particles from being deposited on thepatterning device MA.

In use, the pellicle 19 is positioned at a distance from the patterningdevice MA that is sufficient that any particles that are incident uponthe surface of the pellicle 19 are not in the focal plane of theradiation beam B. This separation between the pellicle 19 and thepatterning device MA, acts to reduce the extent to which any particleson the surface of the pellicle 19 impart a pattern to the radiation beamB. It will be appreciated that where a particle is present in the beamof radiation B, but at a position that is not in a focal plane of thebeam of radiation B (i.e., not at the surface of the patterning deviceMA), then any image of the particle will not be in focus at the surfaceof the substrate W. In some embodiments, the separation between thepellicle 19 and the patterning device MA may, for example, be between 2mm and 3 mm (e.g. around 2.5 mm). In some embodiments, as described inmore detail below, a separation between the pellicle 19 and thepatterning device may be adjustable.

FIG. 2 is a schematic illustration of the pellicle assembly 15 and thepatterning device MA in cross-section and in more detail. The patterningdevice MA has a patterned surface 24. The pellicle frame 17 supports thepellicle 19 around a perimeter portion of the pellicle 19. The pellicleframe 17 may include an attachment mechanism 22 configured to allow thepellicle frame to be removably attachable to the patterning device MA(i.e. to allow the pellicle frame to be attachable to and detachablefrom the patterning device MA). The attachment mechanism 22 isconfigured to engage with an attachment feature (not shown) provided onthe patterning device MA. The attachment feature may, for example, be aprotrusion which extends from the patterning device MA. The attachmentmechanism 22 may, for example, comprise a locking member which engageswith the protrusion and secures the pellicle frame 17 to the patterningdevice MA. It is noted that while referred to as a pellicle frameherein, the pellicle frame 17 may be referred to elsewhere as a pellicleborder. Additionally, the pellicle frame 17 (or “border”) may beattached to the mask via a further pellicle frame. For example, withreference to FIG. 4B of Patent Application No. WO 2016079051 A2, thereis shown an arrangement comprising a pellicle membrane 19, a borderportion 20 (which may, in some arrangements, correspond to the general,but not the specific, component at the position of the “frame 17” of thepresent application) and an additional frame 17 to be attached to thepatterning device MA via attachment mechanisms as shown in FIGS. 12 and31 of WO 2016079051. A plurality of attachment mechanisms and associatedattachment features may be provided. The attachment mechanisms may bedistributed around the pellicle frame 17 (e.g. two on one side of theframe and two on an opposite side of the frame). Associated attachmentfeatures may be distributed around the perimeter of the patterningdevice MA.

A contamination particle 26 is schematically shown in FIG. 2. Thecontamination particle 26 was incident upon the pellicle 19 and is heldby the pellicle 19. The pellicle 19 holds the contamination particlesufficiently far from the patterned surface 24 of the mask MA that it isnot imaged onto substrates by the lithographic apparatus LA.

A pellicle assembly according to an embodiment of the invention mayallow a mask pattern (on the patterning device) to be provided whichremains substantially defect free during use (the mask pattern isprotected from contamination by the pellicle).

The pellicle assembly 15 may be constructed by depositing the pellicle19 (which may be made of, for example polysilicon (pSi)) directly on topof a substrate which is to provide the frame 17. The substrate may be,for example, a silicon wafer. After depositing of the film of thepellicle 19, the substrate may be selectively back-etched to remove acentral portion of the substrate and leave only an outer perimeter toform the frame 17 to support the pellicle 19. The pellicle 19 may have athickness which is, for example, of the order of 15 to 50 nm.

The pellicle 19 requires a level of “pre-stress” (i.e. a level of stresswhich is present within the pellicle 19 when not in use and thereforenot subject to radiation and gas pressure during a scanning operation).The pre-stress within the pellicle 19 allows the pellicle 19 towithstand pressure differences brought about by changes in temperatureand gas pressure during scanning operations. However, where thepre-stress is too large, this will reduce the overall lifetime of thepellicle assembly 15. As such, it is desired to give the pellicle 19 aminimum pre-stress (in the depicted x-y plane) in order to limit thedeflection (in the y direction) at a given pressure on the pellicle 19.When deflection of the pellicle 19 is too large in the y direction, thepellicle 19 might break and/or touch other components in the surroundingarea of the pellicle 19.

The pre-stress of the pellicle 19 is preferably limited since the stressof the pellicle 19 is preferably significantly below the ultimatetensile stress or yield strength of the material from which the pellicle19 is formed. The margin between pre-stress and the ultimate tensilestress should be as large as possible to increase the lifetime andreliability of the pellicle 19.

Pre-stress may be incorporated in the pellicle 19 through one or moremechanisms including stoichiometry, hydrogenation, control of crystalsize, doping and selective mismatch of the coefficient of thermalexpansion (CTE) during depositing of the pellicle 19 onto the wafer 17.The pellicle 19 may also comprise multiple layers (e.g., capping layersto protect the pellicle 19 from, for example, hydrogen radicals). Due toCTE mismatches between layers of the pellicle 19 parts of the pellicleassembly 15 and the process of heteroepitaxy by which the differentlayers of the pellicle 19 may be deposited, the stresses in each layerof a multi-layer pellicle 19 may be different.

The use of stoichiometry, hydrogenation, crystal size selection (i.e.number of grain boundaries), doping and others can all impact thechemical stability of the layers of the pellicle 19. Generally, whilethe process used to manufacture a pellicle 19 may result in a desiredpre-stress (where the desired pre-stress is relatively high), use of theabove outlined techniques are such that the pellicle 19 may not have themost stable configuration from a thermo-chemical perspective. Forexample, a pellicle constructed from molybdenum disilicide (MoSi₂)oxidizes exponentially faster as a function of pre-stress. Moreover, itis generally difficult to obtain a pre-stress below a value of 600 MPain a complementary metal-oxide-semiconductor (CMOS) based manufacturingprocess using the methods outlined above. In some embodiments, it isdesirable to obtain a pellicle 19 with a pre-stress of the order ofapproximately 100-200 MPa.

Additionally, the pellicle frame 17 is generally inflexible compared tothe pellicle 19, resulting in internal stresses in addition to thedesired pre-stress. As such, a pre-stress of e.g. 100 MPa within thepellicle 19, may be in reality a pre-stress of 100 MPa±a deviationcaused by the pellicle frame 15. In other words, some parts of thepellicle 19 may be under a higher stress than others, resulting inpoints of greater mechanical weakness in the pellicle 19.

An average pre-stress within a pellicle 19 is generally the same in boththe depicted x- and y-dimensions. In an embodiment, it is desired toprevent wrinkles with high (of the order of 300 mrad) angles (betweenthe x-y plane and z-dimension). One way to reduce local wrinkles withhigh angles is by reducing the pre-stress in the scanning (x) direction.The stress in scanning (x) direction only need be a small amount largerthan the Poisson ratio v (approximately 0.17-0.25 for MoSi2) multipliedby the stress in y-direction σ_(y) to prevent buckling during rest—i.e.:

σ_(x) = V^(*)σ_(y).

During exposure, the stress in the non-scanning (y) direction σ_(y) willdecline approximately ten times faster than the stress in the scanning(x) direction σ_(x). However, because σ_(x) will be much lower thanv*σ^(y), the buckle modes that can occur on the pellicle 19 within theexposure slit will be of a lower order, hence the chances of local highwrinkle angles are also lower. The lowest order buckle mode is oneperiod of a sine, i.e. a single wrinkle. It takes energy to pull thepellicle 19 into a higher order buckle mode (i.e. such that the pellicle19 contains multiple wrinkles). The more energy that is present, thehigher the chance that locally, due to e.g. an imperfection in thepellicle 192, the pellicle 192 will be pulled into a high angle(locally). As such, in embodiments, using techniques described herein,the amount of pre-stress in the scanning direction may be reduced.

It is desirable to decouple the internal microstructure of the pellicle19 from the pre-stress within the pellicle 19. That is, it is beneficialto control the pre-stress in the pellicle 19 mechanically (orextrinsically), not through the intrinsic microstructure of the pellicle19. This allows the pellicle 19 to be manufactured so as to be aschemically inert as possible, while still allowing for tuning of thepre-stress within the pellicle 19 to a desired level.

FIG. 3 schematically illustrates stages of manufacture of a pellicleassembly 35 in accordance with an embodiment. The pellicle assembly 35is manufactured by etching two notches 36, 37 into a substrate 38 (e.g.,a silicon wafer) that will provide a frame, as shown in FIG. 3a . Apellicle 19 is deposited onto the notched substrate 38 as shown in FIG.3b . The substrate 38 is then back-etched to provide the pellicleassembly 35 having a frame 39, as shown in FIG. 3c . After back etching,the notches 36, 37 result in the frame 39 comprising two expansionstructures in the form of leaf springs (e.g. v-shaped formations) 36′,37′. The leaf springs 36′, 37′ reduce the pre-stress of the pellicle 19in the x-dimension (i.e. the scanning direction).

The length and thickness of the leaf springs 36′, 37′ may be controlledby adjusting the depth of the notches 36, 37 (i.e. their extent in the zdimension) and their position on the x-y plane of the substrate 38. Inthis way, the reduction in stress provided by the leaf springs 36′, 27′may be controlled. Further, the stiffness of the leaf springs may beadjusted independently in both the scanning (x) and non-scanning (y)dimensions. For example, as depicted in FIG. 4, notches may bepositioned on a substrate to provide a lower stress in the scanningdirection (x) than in the non-scanning direction (y). In particular, inFIG. 4, notches 40, 41 are positioned closer (in the x-dimension) to adesired image field 42, thereby resulting a relatively flexible leafspring, while notches 43, 44 may be positioned further from the desiredimage field 42 (in the y-dimension) to create stiffer leaf springs.

FIG. 5 is a graph showing how the stress in a pellicle varies as afunction of the stiffness of a leaf spring (such as the leaf springs36′, 37′). It can be seen from FIG. 5 that by varying the stiffness of aleaf spring 36′, 37′, it is possible to control, within wide bands, thepre-stress within a pellicle.

The use of one or more expansion structures can therefore increase themargin between the pre-stress within the pellicle after production andthe ultimate tensile stress of that pellicle, thereby improving thelifetime and reliability off the pellicle. Additionally, in the event ofcatastrophic failure of the pellicle, the reduced tension achievedthrough the expansion structure means that there is less elastic energystored within the pellicle. As such, there is a lower likelihood thatbreakage of the pellicle will result in pieces of the pellicle would beemitted into other areas of the lithographic apparatus.

It will be appreciated that while expansion structures comprising asingle notch are depicted in FIGS. 3 and 4, expansion structures havingadditional notches and/or other patterns may be etched into thesubstrate from which the pellicle frame is formed so as to provideexpansion structures of differing properties. In this way, pellicleshaving differing pre-stresses may be provided. In some embodiments,additional layers may be utilized in the manufacture of pellicleassemblies in order to provide more complex expansion structures. Forexample, FIG. 6 schematically illustrates stages of manufacture of apellicle assembly 55 (FIG. 6d ). The pellicle assembly 55 ismanufactured by etching serrations 51, 52 into a substrate 53 (e.g., asilicon wafer). A spring layer 54 is deposited onto the substrate 53such that the spring layer 54 is received within and takes the form ofthe serrations 51, 52. The spring layer 54 may be formed of, forexample, metals such as copper, gold, silver, etc. In other embodiments,the spring layer 54 may be formed from other materials such as SiOx, SiNor MoSi. A shield layer 56 is deposited between the substrate 53 and thespring layer 54 over a desired image field. The shield layer 53 need notbe deposited within the serrations 51, 52. Removal of the shield layer53 after depositing the spring layer 54 results in the arrangementillustrated in FIG. 6c , in which the spring layer 54 is restricted tothe areas generally within the serrations 51, 52. A pellicle layer 57 isdeposited onto the spring layer 54 and the substrate 53. Back-etching ofthe substrate 53 provides the pellicle assembly 55 having two expansionstructures 58, 59. In addition to the properties of the serrations 51,52 (e.g., depth, width) the properties of the spring layer 54 may beselected to obtain a desired pre-stress within the pellicle 57, andpellicle frames with very low stiffness levels may be achieved. It willbe appreciated that while a single spring layer 54 is depicted in theexemplary arrangement of FIG. 6, in other embodiments more than onespring layer may be provided. Where more than one spring layer isprovided, spring layers of different materials may be provided.

In some embodiments, the substrate and/or spring layer(s) may be etchedso as to isolate the expansion structures from one another. In atheoretically ideal arrangement, a pellicle may be suspended from apellicle frame by way of an infinite number of expansion structures(e.g., springs), each expansion structure having no internal connectionwith any other of the expansion structures. While it is not possible toprovide an infinite number of expansion structures, it will beappreciated from the teaching herein that different etching patterns maybe utilized within the general techniques described above to isolateeach expansion structure from each other expansion structure. FIG. 7schematically illustrates a top-view of a pellicle frame 75, in which aframe 70 has been etched to provide a plurality of isolated expansionstructures in the form of springs 71, supporting a pellicle 72.

While the above examples have described expansion structures whichoperate in a single dimension, expanding and contracting along thex-axis in FIGS. 3 and 6, in other embodiments expansion structures maybe formed which act in a plurality of dimensions so as to control stresswithin the pellicle in both x and z dimensions. FIG. 8 is a photographshowing a perspective view of a substrate that has been etched toprovide expansion structures which act in both the depicted x and zdimensions.

In an embodiment, the pellicle frame may be etched to provide what isreferred to as a “herringbone” pattern, which comprises a saw-tooth-like(or zig-zag) structure in the x-y plane, the z-y plane and the z-xplane. FIG. 9a is a photograph illustrating a membrane 80 having outerlimits schematically depicted in dashed outline 81. Referring to FIG. 9b, the membrane 80 has been folded to provide a herringbone structure80′. It can be seen that with the herringbone structure 80′, themembrane 80 is smaller than the outer limits 81.

The herringbone structure may be utilized in a plurality of ways withina pellicle assembly. In one embodiment, the herringbone structure isapplied to an outer portion of a pellicle assembly. Referring to FIG.10, a pellicle assembly 90 comprises a pellicle 91 having an image field(or center) portion 92 and an expansion structure in the form of aherringbone portion 93 on a frame (or edge) portion of the pellicleassembly 90. In FIG. 10, the herringbone structure is schematicallyillustrated with hatching. The herringbone patterned edge portion 93 maybe constructed according to the techniques described above withreference to FIG. 6, for example using a different etching pattern.

In another embodiment, the entire surface of a pellicle may be formed soas to comprise a herringbone structure. Referring to FIG. 11, there isshown a pellicle assembly 100 in which an entire pellicle 101, includingan image field portion 102, comprises a herringbone structure(schematically illustrated with hatching). By forming the entirepellicle 101 with a herringbone structure as in the exemplaryarrangement of FIG. 11, the entire surface of the pellicle 101contributes to the reduction in pre-stress across the pellicle 101. Intests, a pellicle with a herringbone pattern has been modelled using thefinite element method, the results of which have shown that an averagestress within a pellicle may be reduced by more than a factor of 10,while the maximum stress experienced by any part of the pellicle isreduced by more than a factor of 6, in comparison to a pellicle withouta herringbone structure.

Where a herringbone structure is applied across the entire surface ofthe pellicle (or at least the majority of the surface of the pellicle),the pellicle may be formed so as to have a total pre-stress of zero Pa.In an exemplary embodiment, the out-of-plane structure (i.e. the extentof the expansion structure(s) in the z-dimension) is of the order of 0.1mm-0.5 mm and the herringbone pattern has a spatial frequency of 0.2mm-1 mm. In such an embodiment, since the additional surface areaprovided by the herringbone structure (compared to a flat pellicle) isorders of magnitude more than the shrinkage of the pellicle duringproduction, the pellicle will have no pre-stress after construction.Surprisingly, while a pellicle with no pre-stress would normally sagunacceptably during use (i.e. sagging which leads to particles on thepellicle being brought into the focal plane), it has been found that aherringbone structure has an additional benefit in that it introducessignificant bending stiffness to the pellicle compared to a flatpellicle, in at least in one direction. Referring to FIG. 12, a pellicle120 having a herringbone structure is illustrated. The herringbonestructure of the pellicle 120 provides relatively high bending stiffnessfor bending around an axis 121, leading to reduce sagging of thepellicle 120, with a lower bending stiffness around axis 122. Thebending stiffness introduced by the herringbone structure suppressesout-of-plane deviations (e.g. sagging in the z-dimension) during use.

The use of one or more expansion structures comprising herringbonestructures, either across an entire pellicle or on an outer (e.g.,frame, edge or non-image field) portion of a pellicle can thereforeincrease the margin between the pre-stress within the pellicle afterproduction and the ultimate tensile stress of that pellicle, therebyimproving the lifetime and reliability off the pellicle. Additionally,in the event of catastrophic failure of the pellicle, the reducedtension achieved through the herringbone structure means that there isless elastic energy stored within the pellicle. As such, there is alower likelihood that breakage of the pellicle will result in pieces ofthe pellicle would be emitted into other areas of the lithographicapparatus.

A herringbone pattern may be formed within a pellicle using the methodsgenerally set out above. That is, the herringbone pattern may first beetched into a substrate (e.g., a silicon wafer) prior to depositing ofthe pellicle on the substrate and prior to back-etching of thesubstrate. In comparison to the methods described above with referenceto FIGS. 3 and 6, to provide a herringbone across the entire surface ofthe pellicle, the entire surface of the substrate may be pre-etched,rather than only an outer portion of the substrate which is to form thepellicle frame. In other embodiments, stress in the pellicle may betuned by introduction of localized portions of herringbone patterns onlywhere it is expected that the pellicle will be subjected to a heat loadduring use.

In an embodiment, stress within a pellicle may be controlled throughmechanical bending of the substrate prior to depositing the pelliclematerial onto the substrate. For example, with reference to FIG. 13a , asubstrate 130 may be mechanically bent into a bow shape. A pellicle 131may be deposited onto the outer (convex) surface of the bowed substrate130 (FIG. 13b ). Finally, the mechanical load within the substrate 130may be released, thereby introducing mechanical compression into thepellicle 131 (FIG. 13c ). Due to the mechanical compression introducedinto the pellicle 131, the tension within the pellicle 131 followingfurther production steps (e.g. back-etching, annealing, etc.), will belower than if the pellicle 131 was deposited onto an initially flatsubstrate.

Bending of the substrate prior to depositing the pellicle material ontothe substrate may also be used to increase tension. For example, withreference to FIG. 14a , a substrate 140 may be mechanically bent into abow shape. A pellicle 141 may then be deposited onto the inner (concave)surface of the bowed substrate 140 (FIG. 14b ). Finally, the mechanicalload within the substrate 140 may be released, thereby introducingmechanical tension.

The stress that is mechanically added or removed from the pellicle as afunction of curvature radius of the substrate on which the pellicle isdeposited during manufacture may, in some embodiments, be calculatedsimilarly to “Stoney's equation”:

$\sigma_{f} = \frac{E_{s}h_{s}^{2}}{6\;{h_{f}\left( {1 - \upsilon_{s}} \right)}R}$

where E_(s) is the Young's modulus of the substrate, h_(s) is thethickness of the substrate, h_(f) is the thickness of the pellicle,v_(s) is the Poisson ratio of the substrate and R is the radius ofcurvature of the substrate that has been mechanically introduced throughbending. It will be appreciated, however, that the amount of stressadded or removed may be different and may be less.

It is to be understood that control of stress within a pellicle throughmechanical bending is not limited to a single layer and can bereplicated with every additional layer of a pellicle that is depositedon the substrate. In this way, the stress of individual layers of thepellicle may be tuned as desired. Further, use of mechanical bending maybe used to control stress in more than one direction. For example, thesubstrate may be mechanically bent in different directions. A bendingdirection may be different for different layers of a pellicle.

Further, a feedback loop may be provided in which a sensor (not shown)is provided to measure forces that occur during depositing of thepellicle as a pellicle layer grows thicker. The mechanical bending ofthe substrate may then be controlled to guarantee the same stressthroughout the entire thickness of the pellicle layer based on thefeedback. Feedback may be especially beneficial in the first atomiclayers of the pellicle as stress develops differently in the firstatomic layers than in the ‘bulk’ of the pellicle.

It was described above with reference to FIGS. 11, 12 that a herringbonepattern may be created across the entirety of a pellicle surface. In analternative embodiment, other patterns or random “roughness” may beintroduced into the pellicle during manufacture in order to lower thefinal pre-stress of the pellicle. Similarly to the methods describedabove, in order to introduce “roughness” into a pellicle roughness maybe introduced onto the surface of the substrate onto which the pellicleis to be deposited. A number of mechanisms may be used to introduceroughness onto the surface of the substrate. For example, with referenceto FIG. 15, a low temperature plasma-enhanced chemical vapor deposition(PECVD) oxide layer 152 (for example silicon dioxide) may be provided onthe surface of a silicon substrate 150. The PECVD oxide layer 152provides a back “etch-stop” layer on which a pellicle 151 is deposited.Low temperature PECVD oxide has a much larger inherent roughness thansilicon. The pellicle 151 will take on this roughness when it isdeposited on the PECVD oxide layer 152.

In other embodiments, potassium hydroxide (KOH) may be used to etch thesurface of the silicon substrate onto which the pellicle is to bedeposited. In particular, a resist may first be deposited onto thesilicon substrate and the deposited resist patterned with a patternusing lithography. For example, a checkerboard pattern may be formedwithin the resist. After lithographically forming a pattern within theresist, the silicon may be etched (e.g. using KOH). For example, thesilicon may be etched create inverse pyramid shapes, the inverse pyramidshapes providing the desired roughness. The resist may then be removedand the substrate cleaned. The cleaned substrate may be oxidized withthermal oxide to round any sharp edges created during the etching and tocreate an “etch stop”. The pellicle may then be deposited onto theoxidized substrate ready for normal pellicle manufacture.

It will be appreciated that other etchants may be used. For example, inother embodiments, an isotropic dry or wet etch may be used. In anotherembodiment, roughness may be created through isotropic etching ofsilicon dioxide, before removal of the resist.

The addition of “roughness” to the pellicle may be restricted toportions of the pellicle, rather than provided across the entire surfaceof the pellicle. For example, similarly to the way in which a spring iscreated in the embodiment described with reference to FIGS. 3 and 6,roughness may be added to the pellicle along one or more edge portionsof the pellicle, or at other portions of the pellicle. For example,stress in the pellicle may be tuned by introduction of localizedportions of roughness only where it is expected that the pellicle willbe subjected to a heat load during use.

The lateral dimension of features created by addition of roughness orpatterns to the pellicle is preferably larger than the thickness of thepellicle. For example, with reference to FIG. 16, a lateral dimension Xof the features (which are depicted as forming a triangular pattern inthe exemplary arrangement of FIG. 16) is larger than the thickness y ofthe pellicle layer 161.

In some embodiments, the dimensions and configuration of the roughnessadded to the pellicle may be selected such that it providesapproximately between 0.045-0.095% extra surface-area across thepellicle than is provided by a flat pellicle. Once the pellicle isfreestanding (e.g., after other steps of the manufacture of the pellicleassembly such as back-etching) then the stress compared to that providedby a flat pellicle, will be reduced by a function of the extra surfacearea translated through the Young's modulus of the pellicle material.

The pellicle may be attached to the pellicle frame using adhesive.Manually dispensing the small amounts of adhesive necessary to attach apellicle to a pellicle frame may be difficult due to process variationsand operator inaccuracies. Issues may occur, for example, when too muchadhesive is applied (process variations) and the portions of adhesiveare positioned too far to an inner side of the frame. Capillary effectsbetween the frame and the pellicle may result in adhesive travellinginside the enclosed volume between the pellicle and the patterningdevice MA. Adhesive inside the enclosed volume can result in seriouscontamination of the patterning device.

FIG. 17 schematically depicts a pellicle frame 171. The pellicle framecomprises a frame border with an inner edge 174. Beyond the inner edge174, a central portion of pellicle is not in direct contact with theframe. A circular boundary 172 is provided on the frame border. Thecircular boundary 172 has been engraved within the material of the frame(which may be, for example, silicon). It has been found that byproviding engraved circles on the frame border, the circles provide acentering function for adhesive supplied in a central area of thecircus. A droplet of adhesive with high contact angle starts wettingimmediately after dispensing. The circular boundary 172 acts as barriercausing the adhesive to self-center within the boundary 172.

An additional boundary 173 is also provided. In the depicted embodiment,the additional boundary 173 takes the form of a straight line engravedinto the frame 171. The additional boundary 173 acts as a furtherbarrier in the event that too much adhesive is applied. FIG. 18a isphotograph showing the centering effect of the circular boundary 172,where it can be seen that adhesive 180 has formed a generally centeredcircular dot of adhesive which is generally concentric with the circularboundary 172. In the depicted embodiment, the diameter of the circularboundary 172 is of the order of 1.4 mm.

The circular boundary 172 and the additional boundary 173 may beprovided by etching the substrate. For example, the boundaries 172, 173may be laser etched into a silicon substrate.

FIG. 18b is a photograph showing a situation in a dot of adhesive 181comprising too large a volume added to the pellicle frame 171. In FIG.18b , it can be seen that the additional boundary 173 acts as a furtherbarrier, preventing the adhesive 181 from spreading beyond the internalborder 174 of the frame where it can contaminate the enclosed volumebetween the pellicle and the patterning device MA.

The provision of a circular boundary 172 and/or an additional boundary173 significantly reduces the risk of adhesive reaching the enclosedvolume and improves the ease and accuracy of dispensing of adhesive,which may be performed manually.

It will be appreciated that the pellicle frame to which the pellicle isattached is relatively stiff in comparison to the pellicle itself. Thestiffness of the frame, in combination with the pre-stress that is addedto the pellicle, as described above, can cause problems with wrinklingof the pellicle when temperatures rise during scanning operations.During scanning, temperatures may rise by up to several hundreds ofdegrees C.

In an embodiment, a mechanism is provided to translate the pellicleframe in directions towards and away from the patterning device MA, soas to close and open the area between the pellicle and the patterningdevice MA. Referring to FIG. 19, a pellicle assembly 190 is showntogether with the patterning device MA. The pellicle assembly 190comprises a pellicle frame 191 and a pellicle 192. Actuators (not shown)are provided to translate the pellicle assembly along the depictedz-dimension. In FIG. 19a , the pellicle assembly is shown in an openconfiguration, in which the pellicle assembly 190 is spaced apart fromthe patterning device MA. In the open configuration, the pressure in thevolume between the pellicle 192 and the patterning device MA is thesame, denoted P₁ in FIG. 19 a.

It will be appreciated that the actuators may take any appropriate formas will be apparent to persons skilled in the art.

The actuators are configured to allow the pellicle assembly 190 totransition to a closed configuration, by translating the pellicleassembly 190 towards the patterning device MA until the pellicle frame191 is in physical contact with the patterning device MA. In the closedconfiguration, the volume between the patterning device MA and thepellicle 192 is physically separated from the external environmentsurrounding the patterning device MA. The pellicle frame 191 maycomprise sealing members (not shown) on surfaces 193 which are arrangedto contact the patterning device MA. The patterning device MA maycomprise corresponding sealing members (not shown) for interfacing withthe pellicle frame 191.

In FIG. 19a , the pressure in the enclosed volume between the pellicle192 and the patterning device MA is still the same pressure, P₁ as thepressure around the patterning device MA.

During a scanning operation, the pellicle assembly 190 may betransitioned to the closed configuration and gasses may be flushed fromthe environment surrounding the patterning device MA and the pellicleassembly 190, thereby lowering the pressure in the environmentsurrounding the patterning device MA. As depicted in FIG. 19c , afterflushing the surrounding area, the pressure in the volume between thepatterning device MA and the pellicle 192 is greater than the pressure(denoted P₂ in FIG. 19c ) surrounding the patterning device MA. As such,the pellicle 192 may bow away from the patterning device MA in thez-dimension.

Prior to transitioning the pellicle assembly 190 from the openconfiguration to the closed configuration, the ambient pressuresurrounding the patterning device MA (denoted P₁ in FIG. 19a ) may beselectively adjusted. For example, in some embodiments, the pressure maybe brought to approximately 2 Pa through the introduction of a gas, suchas, for example, hydrogen gas.

By increasing the pressure between the patterning device MA and thepellicle 192, a thermal connection to the patterning device is improved.This allows for greater cooling through conduction through the gas inthe volume between the patterning device MA and the pellicle 192.Pressure may also be increased from the other side of the pellicle 192,in order to thereby decrease the pressure difference over the pellicle192. In this way, the pressure difference over the pellicle 192, andtherefore controlling the tension in the pellicle 192.

By ensuring that the tension in the pellicle 192 induced through thedifference in pressures P₁, P₂ is greater than the tension in thepellicle 192 induced by the frame 191, the pellicle 192 will wrinklesignificantly less when subjected to a high heat load during scanningoperations. Additionally, by closing the volume between the pellicle 192and the patterning device MA, continuation cannot enter that volumeduring scanning operations.

In the embodiments described above, it is indicated that the substrateonto which the pellicle is deposited may be a crystalline silicon wafer.The bias within the art for the use of crystalline silicon as asubstrate for pellicles has a number of reasons, including that theindustry has vast experience in processing crystalline silicon wafers.Additionally, the coefficient of thermal expansion (CTE) of crystallinesilicon wafers (2.6 um/m/K) matches the CTE of polycrystalline pelliclematerials (approximately 4 um/m/K), such that relatively little thermalstress is introduced during the fabrication process. Crystalline silicontherefore quickly became the default surface onto which pellicles aregrown, even where pellicles are constructed from materials havingdifferent CTEs than polycrystalline pellicle materials.

In some embodiments, pellicles may be constructed from MoSi₂, havingCTEs of approximately 8 um/m/K or graphite-based materials, with CTEs ofapproximately 1 um/m/K. As described above, during annealing at hightemperatures (e.g., approximately 800 degrees Celsius) to improve themicrostructure of the pellicle, the internal stresses within thepellicle assembly are small, (e.g., close to zero). However, duringcool-down (e.g. to room temperature), large CTE mismatches areintroduced between the pellicle and the substrate, resulting in thebuild-up of stresses, contributing to the large pre-stress within thepellicle assembly, as described above.

In an embodiment, the CTE of the pellicle frame is adjusted by creatinga pellicle frame that is formed from a composite material. Referring toFIG. 20a , there is depicted a silicon wafer 200 that is to form thepellicle frame of a pellicle assembly. It will be appreciated that inother embodiments, the substrate may be a different material.

A border region 201 is defined on the substrate 200, defining the edgesof a pellicle frame. While the border region 201 is shown as having agenerally rectangular shape in FIG. 20a , it is to be understood thatother shapes may be used. For example, a circular shaped pellicle framemay introduce less stress concentrations within the pellicle, and maytherefore be particularly beneficial in some embodiments. Portions ofsilicon from within the border region 201 of the substrate 200 areremoved to form a region 202 containing one or more areas for receipt ofanother material. The removed areas are filled with a second (or filler)material having a different CTE to silicon to form a composite region203. Construction of a pellicle assembly using the substrate 200 maythen proceed as normal. The composite region 203 ensures that thepellicle frame of a pellicle assembly constructed using the substrate200 has a desired CTE. The CTE may be adjusted by selecting the fillermaterial and by selecting the way in which the filler material isdistributed within the border region 201.

For example, in one embodiment, silicon may be removed from the borderregion 201 in a continuous band 210 as depicted in FIG. 21a . In analternative embodiment, the border region 201 may be perforated todefine a plurality of perforations 215. The filler material may then beapplied so that it fills the perforations. It will be appreciated thatthe arrangements depicted in FIGS. 21a, 21b are merely exemplary andthat material may be removed from the border region 201 in anyarrangement.

In an embodiment, after filling, the filler material may partially orcompletely surround the border region. FIG. 22 shows a cross-sectionthrough a pellicle assembly 223 in which filler material 220 is residentwithin and around perforations within a silicon substrate 221, therebysurrounding the silicon substrate 221. Together, the silicon substrate221 and the filler material 220 form a pellicle frame 222. In FIG. 22,the substrate has been back-etched to form the pellicle assembly 223having a pellicle 224.

Using a filler material, the CTE of the pellicle frame may be eitherdecreased or increased so as to increase or decrease the pre-stresswithin the pellicle.

The filler material may be any appropriate material that has a differentCTE to the substrate material onto which the pellicle will be deposited(in an example, silicon) and from which the pellicle frame will beformed. By way of example only, the filler material may be a materialhaving a relatively high CTE. Preferably, the filler material is aductile material. Preferably, the filler material is a material thatdoes not outgas. In an example embodiment, the filler material is ametal. In a particular example, the filler material is aluminum ormolybdenum, for example. In an embodiment, different amounts of thesubstrate may be removed at different parts of the border portion. Inthis way, additional, localized, control may be achieved over thepre-stress within the pellicle assembly.

As described above, once the filler material has been added to thesubstrate, all other processing steps to manufacture the pellicleassembly may be beneficially unchanged, such that the use of a compositeframe may be easily introduced into an existing manufacturing process.In an embodiment, rather than utilizing the substrate to form thepellicle frame, a pellicle frame may be bonded pellicle in a separateoperation. The substrate may then be completely etched (such that nopart of the substrate remains in the pellicle assembly). Constructionstages of an example pellicle assembly 230 are depicted in FIG. 23, inwhich a pellicle 231 has been deposited onto substrate 232. Thesubstrate may be, for example, crystalline silicon. Once the pellicle231 has been deposited onto the substrate 232, the pellicle may beannealed as per a conventional pellicle manufacturing method. Afterannealing, however, a frame 233 may be bonded to the pellicle 231. Theframe 233 is bonded to the opposite side of the pellicle to the sidewhich is in contact with the substrate 232. The substrate may then beentirely back-etched to remove the substrate, leaving the pellicleassembly 230 comprising the pellicle 231 and the frame 234. In this way,the pre-stress within the pellicle after manufacture is dependent uponthe difference in CTE between the pellicle 231 and the frame 234 and thetemperature at which the bonding of the frame 234 to the pellicle 231occurred.

It will be appreciated that the term “bonded” as used herein is used todifferentiate the process by which the pellicle is deposited onto thesubstrate. That is, while in prior arrangements, the pellicle adheres tothe substrate after it has been deposited on the substrate, this isdifferent to the present arrangement in which the frame is bonded to thepellicle after the pellicle has been annealed.

The frame 234 may be constructed from any material, such that the CTE ofthe frame 234 may be selected in dependence upon the desired propertiesof the pellicle assembly such as pre-stress. Further, the method (andtherefore the temperature) of bonding the frame 234 may be selected tofurther obtain desired properties of the pellicle assembly 230. Forexample, in one embodiment, the pellicle frame 234 may comprise alithium-aluminosilicate glass-ceramic such as ZERODUR®, from Schott AG.ZERODUR has a very low CTE and therefore little to no expansion,resulting in very low forces exerted on the pellicle and on thepatterning device MA.

In an embodiment, the frame 234 may comprise silicon and it may bedesired to provide a pre-stress within the pellicle 231 in the range of100 to 250 MPa, for example approximately 200 MPa. In order to achieve apre-stress of approximately 200 MPa, the frame 234 may be bonded to thepellicle 231 at a temperature below approximately 160 degrees Celsius.For example, bonding techniques such as optical contact bonding andhydrogen bonding may be used, which operate at temperatures of betweenapproximately 20 to 25 degrees Celsius. Other example bonding techniqueswhich may be used with a silicon frame to obtain a pre-tension ofapproximately 200 MPa include gold diffusion bonding (operating at atemperature of approximately 50 degrees Celsius) and anodic bonding(operating at a temperature of approximately 160 degrees Celsius).

It will be appreciated that where a different amount of pre-stress isdesired within the pellicle 231, different bonding temperatures may beselected. FIG. 24 is a graph showing how the pre-stress within a MoSipellicle increases with the temperature at which a silicon frame isbonded to the pellicle. It will be appreciated that the desiredpre-stress may depend on a large number of factors including, forexample, the composition of the pellicle 231. By way of example, in anembodiment using a graphite-based pellicle 231, a pre-stress in a rangeof from 300 MPa to 450 MPa, for example approximately 400 MPa may bedesired and the material of the pellicle frame 234 and the temperatureof the bonding may be selected accordingly. In particular, thepre-stress a within the pellicle 231 at a given temperature T may begiven by:

σ = −E_(pellicle)((CTE_(pellicle) − CTE_(frame))/(1 − V_(pellicle)))(T − T_(bonding))

where E_(pellicle) is the Young's modulus of the pellicle,CTE_(pellicle) is the CTE of the pellicle, CTE_(frame) is the CTE of thepellicle frame, v_(pellicle) is the Poisson ratio of the pellicle andT_(bonding) is the temperature of bonding.

The method described with reference to FIG. 23 may be additionallyadvantageous in that the frame may be constructed in any size and shapedesired. In other methods of constructing a pellicle assembly, thepellicle frame formed from the etched substrate onto which the pellicleis grown may be assembled onto a further frame for placement on thepatterning device. The method described with reference to FIG. 23advantageously avoids further construction (e.g. gluing, assembly), andreduces the number of interfaces between components, thereby reducingthe generation of particles which may cause contamination and reducingstresses exerted on the pellicle 231 during manufacture. That is, thepellicle frame may be formed from a single piece, rather than from aresidual substrate border and the addition of a further frame element.Additionally, construction of the frame 234 may be more straightforwardthan for other pellicle assembly construction techniques. For example,the pellicle frame 234 may be machined and may be machined from a singlepiece of frame material.

In an embodiment, the pellicle frame 234 may be coated with a chemicallyinert coating prior to bonding to the pellicle 231. In this way,outgassing of the pellicle frame 234 may be reduced or eliminated.

In an embodiment, the entire pellicle frame 234 may be coated with achemical coating that is resistant to the etching used to etch thesubstrate 232. The entire pellicle assembly 230 may then be constructedand cleaned before etching the substrate 232, thereby increasing yield.

In the above example described with reference to FIGS. 23 and 24, it isdescribed that the frame 234 may comprise silicon. In other embodiments,however, the frame may comprise a material that is not silicon. Forexample, then frame 234 may comprise at least one of aluminum, titanium,beryllium, aluminum nitride, Zerodur®, silicon oxide and siliconcarbide. The choice of frame material may be selected depending on thebonding temperature and a desired pellicle pre-stress.

In an embodiment, a pre-stress of a pellicle may be selected so that thepellicle wrinkle in a particular manner (e.g. having wrinkles of aparticular desired dimension) in order to improve performance of thelithographic apparatus. With reference to FIG. 25, there is shown apatterning device MA which is protected by a pellicle 302. A radiationbeam 304 passes through the pellicle 302 and is incident on thepatterning device MA. A patterned radiation beam 306 reflects from thepatterning device MA, passes through the pellicle 302 and is incident ata desired area of a substrate 308. The pellicle 302, however, is notcompletely transparent to the radiation beam 304. A portion of theradiation beam 304 is absorbed by the pellicle 302 while a portion 310is reflected from the pellicle 302. The reflected portion 310 may be,for example, of the order of 0.4% of the radiation beam 304. Thereflected portion 310 does not follow the same path as the patternedradiation beam 306 and is therefore not incident on the desired area ofthe substrate 308. Further, as the reflected portion 310 never interactswith the patterning device MA, the reflected portion 310 is notpatterned with the desired pattern. As such, the reflected beam 306 cancause an overlay errors and/ or reduced contrast.

In an embodiment, pellicle properties are selected so as to cause thedevelopment of wrinkles which reflect at least a portion of the incidentradiation beam along a path of propagation that does not intersect thesubstrate. FIG. 26 shows an arrangement similar to that shown in FIG. 25with like components provided with like reference numerals. In FIG. 26,however, a pellicle 402 is configured to wrinkle in such a way so as tocause the reflected portion 310 to follow a path which does notintersect the substrate 308. As such, in the embodiment of FIG. 26, theportion 310 does not result in reduced contrast or overlay errors. Itwill be appreciated that it may not be possible to prevent allreflections from the pellicle 402 onto the substrate (e.g. portionsreflected from peaks of troughs of wrinkles), but that by selecting forappropriate wrinkles, an amount of radiation which is reflected from thepellicle onto the substrate may be reduced.

The quality of the wrinkling of the pellicle 402 may be controlled usingany of the techniques described herein to control the pre-stress presentwithin the pellicle 402. Additionally, the temperature of the pellicle402 may also be controlled. In an embodiment, the pre-stress andtemperature of the pellicle 402 is selected so as to cause wrinkles withan angle α of greater than approximately 35 mrad to a plane defined bythe surface of the pellicle at room temperature. It will be appreciated,however, that the desired angle of the wrinkle may vary in dependenceupon an optical path length between the patterning device and thesubstrate.

In an embodiment, the pellicle 402 is subjected to a pre-stress ofapproximately 240 MPa. In an embodiment, the pellicle 402 may be heatedto an operational temperature (e.g. during a patterning operation) ofbetween approximately 200-450 degrees C. In an exemplary embodiment inwhich the pellicle 402 comprises a pre-stress of approximately 240 MPaand is heated to approximately 450 degrees C., the pellicle 402 mayexhibit wrinkles having a maximum local angle α of up to approximately40 mrad and a wrinkle height Wh of the order of 10 μm. Generally,however, it will be appreciated that the wrinkle height may differ andthat the wrinkle height may depend upon a material composition of thepellicle. In an embodiment in which the wavelength of the radiation beam304 is of the order of 13.5 nm, the pellicle may be configured toexhibit wrinkles with a minimum height of the order of 10 μm. While itmay generally have been expected that wrinkles in the pellicle 402 mayincrease absorption of the incident radiation beam 304, it has beendetermined that any increase in absorption of the radiation beam 304 dueto wrinkles having local angels of less than approximately 300 mrad maybe acceptable.

Pellicles may have a finite time in which they are effective and/orduring which the likelihood of failure is within a desired threshold.For example, for some pellicles, it may be the case that after a certainuse-time, the pellicles become sufficiently likely to fail that it ispreferred to avoid their use. In the event of catastrophic failure ofthe pellicle, the elastic energy stored within the pellicle may resultin breakage of the pellicle which in turn may result in pieces of thepellicle being emitted into other areas of the lithographic apparatus,thereby resulting in downtime of the lithographic apparatus.

In order to allow for a determination to be made as to whether apellicle should be used, an embodiment provides a wirelessly readableand writeable tracking device within the pellicle frame of a pellicle.For example, the pellicle frame may include a near-field communication(NFC) chip (such as an RFID “tag”). For example, the tracking device maybe installed into the pellicle frame during manufacture or mounting ontoa patterning device. The tracking device provided with a serial numberwhich uniquely identifies the pellicle and/or the pellicle assembly. Thetracking device may further store operational data which may be used totrack the operational history of the pellicle. For example, theoperational data may include a number of exposures to which the pelliclehas been exposed, a time of manufacture, a total amount of radiation(e.g., based upon a number of exposures and a known power of eachexposure) and/or any other information which may be used to determinewhether a pellicle has reached an end of its useful lifetime. Thetracking device may also store pellicle specific information, forexample a list of parameter values measured offline (e.g. EUVtransmission, reflection, etc.).

When the pellicle/patterning device combination is loaded into thelithographic apparatus, a transceiver arrangement (comprising, forexample, a single transceiver or a separate receiver and transmitter)provided within the lithographic apparatus may be caused (i.e., by acontroller) to read the serial number of the pellicle and theoperational data. The controller may determine whether one or moreparticular ones of the operational data items exceeds a threshold. Forexample, the controller may determine whether the number of exposures towhich the pellicle has been exposed, or a time since manufacture,exceeds a threshold. If the controller determines that one or more ofthe operational data items exceeds a threshold, the lithographicapparatus may unload the pellicle/patterning device combination. Thecontroller may further cause the transceiver to write data to thetracking chip to mark the pellicle as unusable or to indicate that ithas been refused for use by the particular lithographic apparatus.

If it is determined that the particular operational data items do notexceed their relevant thresholds, the controller tracks the usage of thepellicle during operation. For example, the controller may record thenumber of exposures to which the pellicle is exposed and may cause thetransceiver to write that data to the tracking device. The tracking datamay be written to the tracking device during use or during an unloadingoperation in which the pellicle/patterning device are removed from thelithographic apparatus.

Upon removal from a lithographic apparatus, the tracking device within apellicle/patterning device combination may be read to determine whetherthe pellicle has been marked as unusable, or whether one or more of theoperational data items exceed a threshold. It may then be determinedwhether to replace the pellicle for the patterning device.

FIGS. 27a-27c schematically depict, other arrangements by which apellicle assembly may be strengthened to prevent breakage during use ormanufacture. Referring to FIG. 27a , a pellicle assembly 500 comprises apellicle frame 502 supporting a pellicle 504. The pellicle frame 502may, for example, be manufactured as described above. A tensile layer506 is applied to a “front” or “top” side of the pellicle arrangement500 (i.e. such that the pellicle 504 is disposed between the tensilelayer 506 and the pellicle frame 502). It will be appreciated that thetensile layer 506 may be provide onto the pellicle 504 in anyappropriate manner (such as by any form of deposition or by growing thetensile layer 506 on the pellicle 504).

Viewed in cross-section as depicted in FIG. 27a , the tensile layer 506extends laterally inward beyond (e.g. overlaps or bridges) the inwardedge 508 of the pellicle frame 502. The location of the inward edge 508of the pellicle frame is normally a weak point in the pellicle assemblyand a location at which pellicle assemblies are often subject tomechanical failure. The presence of the tensile layer 506, extendingover the weak point at the edge 508, acts to maintain tension in thepellicle 504 and strengthen the pellicle assembly 500. In this way thetensile layer 506 allows for the pellicle 504 to be subject toadditional pretension than would be possible without the presence of thetensile layer 506. As discussed in detail above, applying appropriateand sufficient levels of pretension to the pellicle 504 improves theoptical properties of the pellicle assembly by preventing wrinkling. Thetensile layer 508 preferably extends inwardly only over a non-opticallyactive portion of the pellicle 504. That is, the tensile layer 508preferable does not extend inwardly to an extent at which it wouldoverlap a beam path of the radiation beam B. As in the arrangementdepicted in FIG. 27a , it is preferable (but not essential) for thetensile layer 508 to have an outward extent from the edge 508 greaterthan the inward extent from the edge 508 to provide greater strength. Inthis way, as depicted in FIG. 27a , the tensile layer 506 comprises afirst portion 506 a which extends inwardly from the edge 508 and asecond, larger, portion 506 b which extends outwardly from the edge 508.

As shown with reference to the pellicle assembly 510 in FIG. 27b , atensile layer 512 may instead be applied to a “bottom” or “underside” ofthe pellicle 504. As with the tensile layer 506 in the pellicle assembly500, the tensile layer 512 of the pellicle assembly 510 extends acrossthe inner boundary between the pellicle frame 502 and the pellicle 504,thereby strengthening the pellicle assembly 510. In the arrangement ofFIG. 27b , a first portion 512 a of the tensile layer 506 extendsinwardly from the edge 508, and a second, larger, portion 512 b extendsoutwardly from the edge 508. Additionally, in the arrangement 510, thesecond portion 512 b extends over a plurality of faces of the pelliclefrom 502. In particular, in the example of FIG. 27a , the second portion512 b extends over all exposed faces of the pellicle frame 504 (e.g. allfaces other than the “top” face to which the pellicle 504 is applied).In this way, the tensile layer 512 is particularly able strengthen thepellicle assembly 510 and allow for greater pretension of the pellicle504.

Further, as depicted in FIG. 27c , a pellicle assembly 514 may compriseboth the tensile layer 506 and the tensile layer 512. The tensile layermay comprise any suitable material. For example, the tensile layer maycomprise molybdenum (Mo), ruthenium (Ru), well adhering metal layers,such as aluminum (Al), titanium (Ti), tantalum (Ta), vanadium (V),chromium (Cr), niobium (Nb), hafnium (Hf), tungsten (W), etc., oxidessuch as TiO2, Ta2O5, etc. nitrides such as TiN, TaN, CrN, SiN orcarbides such as TiC, TaC, SiC, etc. Generally, any material thatadheres sufficiently and may be deposited with sufficient tensile stress(as determined by the particular requirements of the particularapplication) may be used. Further, the tensile layers may comprise amultilayer structure, with each layer comprising a different material.For example, the tensile layer may comprise a capping, protective layeratop a tension providing layer. By way of example only, a tensile layermay comprise a first layer of molybdenum to provide a tension and asecond layer of ZrO2 to act as a protective layer. Additionally, tensilelayers provided on the front (such as the tensile layer 506) may have adifferent structure and composition to tensile layers provided on theback (such as the tensile layer 512). This beneficially allows fordifferent materials to be used to match different operating conditions“above” and “below” the pellicle 504. It will further be appreciatedthat thicknesses of the tensile layer and distances by which the tensilelayers extend either side of the edge 508 may be selected in dependenceupon the particular

Generally, the above-described arrangements provide pellicle assembliesfor which the stress under non-operating conditions (e.g., pre-stress)maybe selected in dependence upon application requirements. Where, forexample, it is desired to provide a lower pre-tension than currentlyachievable, the lifetime of a pellicle may be improved while allowingfor a desired thermo-chemical nature. That is, the above techniquesallow for a separation of the thermo-chemical nature of the pellicle andthe pre-stress present in the pellicle after manufacture of the pellicleassembly. Further, in the event of catastrophic failure of the pellicle,the reduced tension that may be achieved through the above disclosedarrangements is that there is less elastic energy stored within thepellicle. As such, there is a lower likelihood that breakage of thepellicle will result in pieces of the pellicle would be emitted intoother areas of the lithographic apparatus, thereby reducing downtime ofthe lithographic apparatus. Further, while the above describedarrangements are described with respect to pellicles that are used toprotect a patterning device MA, it is to be understood that the abovepellicle assemblies described herein may be used in other applications,both for lithography and more broadly. For example, the above describedarrangements may be used to provide pellicle assemblies for use indynamic gas locks.

In an embodiment, the invention may form part of a mask inspectionapparatus. The mask inspection apparatus may use EUV radiation toilluminate a mask and use an imaging sensor to monitor radiationreflected from the mask. Images received by the imaging sensor are usedto determine whether or not defects are present in the mask. The maskinspection apparatus may include optics (e.g. mirrors) configured toreceive EUV radiation from an EUV radiation source and form it into aradiation beam to be directed at a mask. The mask inspection apparatusmay further include optics (e.g. mirrors) configured to collect EUVradiation reflected from the mask and form an image of the mask at theimaging sensor. The mask inspection apparatus may include a processorconfigured to analyze the image of the mask at the imaging sensor, andto determine from that analysis whether any defects are present on themask. The processor may further be configured to determine whether adetected mask defect will cause an unacceptable defect in imagesprojected onto a substrate when the mask is used by a lithographicapparatus.

In an embodiment, the invention may form part of a metrology apparatus.The metrology apparatus may be used to measure alignment of a projectedpattern formed in resist on a substrate relative to a pattern alreadypresent on the substrate. This measurement of relative alignment may bereferred to as overlay. The metrology apparatus may for example belocated immediately adjacent to a lithographic apparatus and may be usedto measure the overlay before the substrate (and the resist) has beenprocessed.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 4-20 nm, for examplewithin the range of 13-14 nm. EUV radiation may have a wavelength ofless than 10 nm, for example within the range of 4-10 nm such as 6.7 nmor 6.8 nm.

Although it is described above that the source SO may be a laserproduced plasma LPP source, any suitable source may be used to generateEUV radiation. For example, EUV emitting plasma may be produced by usingan electrical discharge to convert fuel (e.g. tin) to a plasma state. Aradiation source of this type may be referred to as a discharge producedplasma (DPP) source. The electrical discharge may be generated by apower supply which may form part of the radiation source or may be aseparate entity that is connected via an electrical connection to theradiation source SO.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the clauses set out below.

1. A pellicle assembly, the pellicle assembly comprising a pellicleframe defining a surface onto which a pellicle is attached;

wherein the pellicle assembly comprises one or more three-dimensionalexpansion structures that allow the pellicle to expand under stress.

2. The pellicle assembly of clause 1, wherein at least one of thethree-dimensional expansion structures is formed within the pellicleframe and thereby imparted to the pellicle.3. The pellicle assembly of clause 1 or 2, wherein at least one of thethree-dimensional expansion structures forms a spring.4. The pellicle assembly of clause 3, wherein the spring comprises atleast one V-shaped formation formed within the pellicle frame.5. The pellicle assembly of any preceding clause, wherein the at leastone structure comprises a plurality of springs formed within thepellicle frame and positioned around a central portion of the pellicle.6. The pellicle assembly of clause 2 or any clause dependent thereon,wherein the at least one spring is a leaf-spring.7. The pellicle assembly of any preceding clause, wherein at least oneof the three-dimensional expansion structures is present on a centralportion of the pellicle.8. The pellicle assembly of clause 7, wherein the frame comprises asubstrate and the pellicle assembly comprises at least one spring layerbetween the substrate and the pellicle, wherein at least one spring isformed within the spring layer.9. The pellicle assembly of any preceding clause, wherein the at leastone three-dimensional expansion structure comprises a herringbonepattern.10. The pellicle assembly of clause 9, wherein the at least onethree-dimensional expansion structure comprises a herringbone patternthat extends across a majority of a total surface of the pellicle.11. The pellicle assembly of any preceding clause, wherein the at leastone three-dimensional expansion structure results in roughness across asurface of the pellicle.12. A pellicle assembly, the pellicle assembly comprising a pellicleframe defining a surface onto which a pellicle is attached;

wherein the surface comprises at least one adhesive boundary forreducing adhesive spread.

13. The pellicle assembly of clause 12, wherein the at least oneadhesive boundary comprises a circular boundary.14. The pellicle assembly of clause 12 or 13, wherein the at least oneadhesive boundary comprises a line boundary.15. The pellicle assembly of clause 14, wherein the line boundary ispositioned adjacent an edge of the frame, the edge of the frame adjacenta central portion of the pellicle.16. The pellicle assembly of clause 14 or 145 as dependent on clause 13,wherein the line boundary is positioned between the circular boundaryand a central portion of the pellicle.17. The pellicle assembly of any one of clauses 12 to 16, wherein the atleast one boundary comprises a groove within the frame.18. The pellicle assembly of any one of clauses 12 to 17, wherein thepellicle assembly comprises adhesive that is substantially concentricwith the circular boundary.19. A pellicle assembly comprising:

a pellicle frame;

a pellicle;

one or more actuators for moving the pellicle assembly towards and wayfrom a patterning device.

20. The pellicle assembly of clause 19, wherein the actuators areconfigured to transition the pellicle assembly between a closedconfiguration in which a substantially sealed volume is formed betweenthe pellicle and a patterning device and an open configuration in whicha volume between the pellicle and the patterning device is in fluidcommunication with an ambient environment.21. A pellicle assembly comprising:

a pellicle frame defining a surface onto which the pellicle is attached;

wherein the pellicle frame comprises a first material having a firstcoefficient of thermal expansion and a second material having a secondcoefficient of thermal expansion.

22. The pellicle assembly of clause 21, wherein the first materialcomprises silicon.23. The pellicle assembly of clause 21 or 22, wherein the first materialcomprises a plurality of perforations and the second material is locatedwithin the plurality of perforations.24. The pellicle assembly of clause 21, 22 or 23, wherein the secondmaterial at least partially surrounds the first material.25. The pellicle assembly of any one of clauses 21 to 24, wherein thesecond material comprises a metal.26. A pellicle assembly comprising:

a pellicle frame defining a surface onto which the pellicle is attached;

wherein the pellicle frame is bonded to the pellicle.

27. The pellicle frame of clause 26, wherein the pellicle is an annealedpellicle and the pellicle frame was bonded to the pellicle afterannealing.28. The pellicle assembly of clause 26 or 27, wherein the pellicle framecomprises a material having a coefficient of thermal expansion (CTE)which is lower than the CTE of silicon.29. The pellicle assembly of clause 26, 27 or 28, wherein the pellicleframe comprises a glass-ceramic material.30. The pellicle assembly of any of clauses 26 to 29, wherein thepellicle frame has been bonded to the pellicle using a bonding procedureoperating at a temperature of less than approximately 160 degreesCelsius.31. The pellicle assembly of any of clauses 26 to 30, wherein thepellicle frame has been bonded to the pellicle using at least one ofoptical contact bonding, hydrogen bonding, gold diffusion bonding oranodic bonding.32. The pellicle assembly of any of clauses 26 to 31, wherein thepellicle frame is formed in a single piece.33. The pellicle assembly of any of clauses 26 to 32, wherein thepellicle frame comprises an inert coating to reduce outgassing.34. The pellicle assembly of any of clauses 26 to 33, wherein thepellicle frame does not comprise silicon.35. The pellicle assembly of any one of clauses 26 to 34, wherein thepellicle frame comprises at least one of aluminum, titanium, beryllium,aluminum nitride, Zerodur®, silicon oxide and silicon carbide.36. A pellicle assembly, the pellicle assembly comprising a pellicleframe defining a surface onto which a pellicle is attached;

wherein when irradiated with a radiation beam for patterning a substrateto be patterned the pellicle comprises one or more wrinkles and ispartially reflective of the radiation beam; and

wherein the one or more wrinkles are configured to reflect a portion ofthe radiation beam away from the substrate to be patterned.

37. The pellicle assembly of clause 36, wherein the pellicle isconfigured such that the wrinkles create a maximum angle of greater than35 mrad to a plane defined by the surface of the patterning device.38. The pellicle assembly of clause 36 or 37, wherein the pellicle isconfigured such that the wrinkles create a maximum angle of less than300 mrad to a plane defined by the surface of the patterning device.39. The pellicle assembly of clause 36, 37 or 38, wherein the pellicleis configured to reflect approximately 0.4% of an incident radiationbeam during use.40. The pellicle assembly of any preceding clause, wherein the pelliclecomprises molybdenum disilicide (MoSi₂) and wherein an average stresswithin the pellicle is in a range of from 100 MPa to 250 MPa at roomtemperature.41. The pellicle assembly of any preceding clause, wherein the pelliclecomprises a graphite-based material and wherein an average stress withinthe pellicle is in a range of from 300 MPa to 450 MPa at roomtemperature.42. A lithographic apparatus arranged to project a pattern from apatterning device onto a substrate, comprising a pellicle assemblyaccording to any preceding clause positioned in the vicinity of thepatterning device to prevent particles from contacting the patterningdevice.43. A method of manufacturing a pellicle assembly comprising:

depositing a pellicle onto a substrate;

bonding a pellicle frame to the pellicle such that the pellicle isbetween the pellicle frame and the substrate;

etching the substrate to leave the pellicle and the pellicle frame.

44. The method of clause 43, wherein the pellicle frame is bonded to thepellicle at a temperature below 160 degrees Celsius.45. The method of clause 43 or 44, further comprising annealing thepellicle after depositing the pellicle onto the substrate and beforebonding the pellicle frame to the pellicle.46. A pellicle assembly, comprising:

a pellicle frame defining a surface onto which a pellicle is attached;

wherein a computer readable and writeable tracking device is provided onor in the pellicle frame.

47. The pellicle assembly of clause 46, wherein the computer readableand writable tracking device is configured to store an identifier of thepellicle and/or the pellicle assembly.48. The pellicle assembly of clause 46 or 47, wherein the computertracking device is configured to store operational data indicating usagehistory of the pellicle and/or pellicle specific properties.49. A lithographic apparatus arranged to project a pattern from apatterning device onto a substrate, comprising a controller comprising aprocessor configured to execute computer readable instructions to causea transceiver arrangement to read from and write to a tracking device ofa pellicle assembly according to any one of clauses 46 to 48.50. The lithographic apparatus of clause 49, wherein the computerreadable instructions are configured to cause the processor to obtainone or more operational data items from the tracking device and todetermine whether the one or more operational data items exceed athreshold.51. The lithographic apparatus of clause 50, wherein the computerreadable instructions are configured to cause the processor, in responseto determining that the one or more operational data items exceed athreshold, to cause the lithographic apparatus to unload the pellicleassembly.52. The lithographic apparatus of clause 51, wherein the computerreadable instructions are configured to cause the processor to cause thetransceiver arrangement to write to the tracking device data indicatingthat the pellicle assembly has been unloaded.53. The lithographic apparatus of clause 50 or 52, wherein the computerreadable instructions are configured to cause the processor, in responseto determining that the one or more operational data items do not exceedthe threshold, to cause the transceiver to record operational data onthe tracking device during use of the lithographic apparatus.54. A pellicle assembly, comprising:

a pellicle frame defining a surface onto which a pellicle is attached;and

one or more tensile layers in mechanical communication with the pellicleand extending inwardly beyond an inner edge of the pellicle frame.

55. The pellicle assembly of clause 54, wherein at least one of the oneor more tensile layers is provided on a top side of the pellicle suchthat the pellicle is positioned between the tensile layer and the frame.56. The pellicle assembly of clause 54 or 55, wherein at least one ofthe one or more tensile layers is provided on an underside of thepellicle.57. The pellicle assembly of clause 54, 55 or 56, wherein the one ormore tensile layers comprises a first tensile layer provided on a topside of the pellicle and a second tensile layer provided on an undersideof the pellicle.58. The pellicle assembly of any one of clauses 54 to 57, wherein atleast one of the one or more tensile layers comprises a first portionextending inwardly from the inner edge of the pellicle frame and asecond portion extending outwardly from the inner edge of the pellicleframe, wherein the second portion is longer than the first portion.59. The pellicle assembly of any one of clauses 54 to 57, wherein atleast one of the one or more tensile layers comprises a multi-layerstructure.60. A lithographic apparatus arranged to project a pattern from apatterning device onto a substrate, comprising a pellicle assemblyaccording to any of clauses 54 to 59 positioned in the vicinity of thepatterning device to prevent particles from contacting the patterningdevice.61. A dynamic gas lock for a lithographic apparatus, comprising apellicle assembly according to any one of clauses 1 to 41, 46 to 48 or54 to 59.

1. A pellicle assembly, the pellicle assembly comprising a pellicleframe defining a surface onto which a pellicle is attached, wherein thepellicle assembly comprises one or more three-dimensional expansionstructures that allow the pellicle to expand under stress relative tothe pellicle frame.
 2. The pellicle assembly of claim 1, wherein atleast one of the three-dimensional expansion structures is formed withinthe pellicle frame and thereby imparted to the pellicle.
 3. The pellicleassembly of claim 1, wherein at least one of the three-dimensionalexpansion structures forms a spring.
 4. The pellicle assembly of claim3, wherein the spring comprises at least one V-shaped formation formedwithin the pellicle frame.
 5. The pellicle assembly of claim 1, whereinthe at least one structure comprises a plurality of springs formedwithin the pellicle frame and positioned around a central portion of thepellicle.
 6. The pellicle assembly of claim 2, wherein the at least onespring is a leaf-spring.
 7. The pellicle assembly of claim 1, wherein atleast one of the three-dimensional expansion structures is present on acentral portion of the pellicle.
 8. The pellicle assembly of claim 7,wherein the frame comprises a substrate and the pellicle assemblycomprises at least one spring layer between the substrate and thepellicle, wherein at least one spring is formed within the spring layer.9. The pellicle assembly of claim 8, wherein the spring layer is etchedso as to provide a plurality of isolated expansion structures.
 10. Thepellicle assembly of claim 8, wherein the spring layer is formed of ametal such as Cu, Au, Ag or of SiOx, SiN or MoSi.
 11. The pellicleassembly of claim 1, wherein the at least one three-dimensionalexpansion structure comprises a herringbone pattern.
 12. The pellicleassembly of claim 11, wherein the at least one three-dimensionalexpansion structure comprises a herringbone pattern that extends acrossa majority of a total surface of the pellicle.
 13. The pellicle assemblyof claim 1, wherein the at least one three-dimensional expansionstructure results in roughness across a surface of the pellicle.
 14. Thepellicle assembly of claim 13, wherein the roughness is generated bymeans of chemical vapor deposition or etching.
 15. The pellicle assemblyof claim 1, wherein the at least one three-dimensional expansionstructure is provided between the pellicle frame and the pellicle. 16.The pellicle assembly of claim 1, wherein the at least onethree-dimensional expansion structure forms a connection between thepellicle frame and the pellicle.
 17. The pellicle assembly of claim 1,wherein the pellicle comprises molybdenum disilicide (MoSi₂) and whereinan average stress within the pellicle is in a range of from 100 MPa to250 MPa at room temperature.
 18. The pellicle assembly of claim 1,wherein the pellicle comprises a graphite-based material and wherein anaverage stress within the pellicle is in a range of from 300 MPa to 450MPa at room temperature.
 19. A lithographic apparatus arranged toproject a pattern from a patterning device onto a substrate, comprisinga pellicle assembly according to claim 1 positioned in the vicinity ofthe patterning device to prevent particles from contacting thepatterning device.
 20. A dynamic gas lock for a lithographic apparatus,comprising a pellicle assembly according to claim 1.